Encapsulated particles that include layers, such as polyurethane layers, and core particles are known in the art. A thickness and external integrity of the polyurethane layers limit a rate at which the core particles are released, e.g. dissolve.
Prior art encapsulated particles tend to exhibit surface defects and issues with inconsistent external integrity such as partial encapsulation of the core particle by the polyurethane layer, inconsistent thickness of the polyurethane layer, and pits and depressions in the polyurethane layer. When polyurethane layers with the surface defects are disposed about core particles, water and other liquids permeate the polyurethane layer and rapidly dissolve the core particle, which is typically undesirable.
In many cases, the surface defects result from the incomplete reaction of a polyol component and an isocyanate and/or problems with an encapsulation process employed to form the polyurethane layer. During the encapsulation process, the polyol component and the isocyanate chemically react to form the polyurethane layer. The polyol component and the isocyanate may have physical properties, such as viscosity, and/or chemical properties, such as polarity, which impede complete encapsulation of the core particle, affect the formation of the polyurethane layer having consistent thickness, and cause pits and depressions to form in the polyurethane layer. The physical and chemical properties of the polyol component, the isocyanate, and a reaction mixture formed therefrom may also cause agglomeration of the core particles during the encapsulation process, which, in turn, causes a decreased yield of encapsulated particles and causes the surface defects to form in the polyurethane layer of the encapsulated particles.
The surface defects in the polyurethane layer may also result from incomplete miscibility between the isocyanate and the polyol. For example, when a polyol component including a non-aromatic polyol is combined with an isocyanate including an aromatic isocyanate, miscibility may be compromised. The non-aromatic polyol may react with the aromatic isocyanate in a partial manner only at an interface resulting in the surface defects, such as the pits and depressions, in the polyurethane layer.
To minimize the impact of the surface defects, multiple layers can be disposed about the core particles. However, forming multiple layers typically requires a time consuming and expensive encapsulation process. As one example, a first polyurethane layer may be disposed about the core particle followed by a second organic wax layer, which may be used to reduce the permeation of water and other liquids through any of the surface defects in the first polyurethane layer and the subsequent rapid dissolution of the core particles.
In addition, agglomeration of core particles typically occurs during encapsulation of the core particles. Agglomeration may be caused by a coating and/or encapsulation process. FIG. 1 is a cross-sectional view of clumped or agglomerated encapsulated particles of the prior art. Agglomeration of the core particles during the encapsulation process tends to reduce encapsulation efficiency, impedes complete encapsulation of the core particle by the polyurethane layer, restricts the formation of the polyurethane layer having consistent thickness, increases the amount of the polyurethane layer required relative to the amount of core particles needed to encapsulate the core particle, decreases a yield of encapsulated particles, and causes pits and depressions in the polyurethane layer of the yield of encapsulated particles. Accordingly, there remains a need to develop an improved polyurethane layer and method of encapsulating core particles.